The present invention relates to photosensitive elements for use in electrophotography and, more particularly, to a new and improved photosensitive element having a photoconductive layer providing good toner transferability, transfer paper separability, and toner cleanability.
Photosensitive elements for use in electrophotography are employed by charging and exposing the surface of a photoconductive layer to form a latent image, developing the latent image with toner, and transferring the developed image to another surface such as paper. The electrophotographic process also includes, subsequent to the above steps, a step of removing electric charges remaining on the photoconductive layer and a cleaning step of removing residual toner remaining on the surface of the photoconductive layer after transfer of the toner image. Since cleanability of the photoconductive layer governs the quality of a next image to be formed, a photoconductive layer of good cleanability is in demand.
Good image reproduction requires that conventional photosensitive elements for use in electrophotography have a very smooth photoconductive layer surface, and hence the photoconductive layer surface has been formed as a polished mirror surface. However, such a photoconductive layer has a high degree of adhesion to transfer paper or toner, resulting in impaired toner transferability, transfer paper separability, and toner cleanability. To avoid this problem, there have been used mechanical toner removal devices such as cleaning blades which impose strong scraping forces on the photoconductive layer surface for removing any residual toner. The mechanical toner removal devices, however, tend to damage the photoconductive layer surface or otherwise shorten the service life of the photosensitive element, with the result that images formed on the surface will have a reduced quality. It has recently been found that roughening the photoconductive layer surface to an appropriate extent is effective in eliminating the above drawback.
Various processes have been proposed to roughen the surface of the photoconductive layer. One method has been to form prescribed ridges and grooves in the surface of an electrically conductive substrate and form a photoconductive layer over the ridges and grooves. According to another process, foreign matter is embedded in the surface of a photoconductive layer. The former method is less feasible industrially since the entire substrate is required to have a uniform roughened surface with no localized blemishes or flaws in order to form images of good quality. The latter process is liable to reduce the service life of the photosensitive body because contact between the photoconductive layer and foreign matter promotes the generation of crystal nuclei of the photoconductive material at the points of contact. Therefore, the photoconductive layer should preferably be made only of a photoconductive material, and it would be most advantageous from the manufacturing standpoint if the surface of the photoconductive layer covering the electrically conductive substrate could be roughened after it has been formed. Prior photosensitive elements made of selenium, however, suffer from crystallization if they are roughened by such a process, e.g., if the surface of the photoconductive layer is roughened by grinding. Such crystallization impairs the electrostatic characteristics of the photoconductive layer.
It is an object of the present invention to provide a photosensitive element for use in electrophotography which has a surface which has been roughened after formation to provide an appropriate roughened finish.